Single crystalline silicon (unicrystalline silicon) is useful in the electronics industry and finds utility particularly in semiconductors and photodetectors. As electronic devices become more sophisticated, the need for high purity, relatively defect free unicrystalline silicon is sought. Indeed, the United States government is actively supporting research programs to produce highly pure, relatively defect-free unicrystalline silicon.
Typical characteristics sought for highly pure, relatively defect free unicrystalline silicon are high resistivities and long lifetimes. The resistivities of semiconductor materials are affected by ionizable impurities, e.g., boron, phosphorus, arsenic, aluminum, heavy metals, etc. The lifetimes of these materials pertain to the time that a carrier in the material stays free and contributes to the electrical conductivity of the material. When the carrier is recombined, its lifetime is terminated. Recombination can occur at imperfections within unicrystalline silicon or by combination with an oppositely charged carrier, e.g., from impurities. Hence, unicrystalline material that is free from defects and ionizable impurities would be expected to exhibit very long lifetimes. It should be recognized that some increase in activities and lifetimes can be obtained by inclusion of materials (dopants) that can neutralize the effect of the ionizable impurities and materials that can trap carriers.
The primary commercially employed methods for producing unicrystalline silicon are the well-known Czochralski technique and the float zone techniques, both of which use polycrystalline silicon as the feed material. In the Czochralski technique, particles of polycrystalline silicon are melted in a crucible and unicrystalline silicon is drawn from the melt. The float zone process does not use a crucible. Rather, a rod of polycrystalline silicon is heated in a zone with unicrystalline silicon being drawn from the heated zone. The float zone technique, since the silicon is not contained within a crucible, is generally thought to be able to provide unicrystalline silicon containing less impurities. Recent efforts, however, are being directed to magnetic Czochralski techniques in which the high temperature silicon melt is maintained localized within the center of the crucible, thereby minimizing contamination from the crucible. Nonetheless, impurities in the parts per trillion (atom basis) range can adversely affect resistivities and lifetimes of unicrystalline silicon and thus even spurious atoms emitted from induction heaters as used in the float zone technique are serious considerations.
The purity of the polycrystalline silicon feedstock will affect the purity of the unicrystalline silicon. Polycrystalline silicon is commercially manufactured by the thermal decomposition of silane or a halogenated silane. The temperatures of the deposition may range from 400.degree. C. to over 1200.degree. C. depending upon whether silane or a halogenated silane is used. Sources of impurities are multifold. For instance, impurities such as boron, phosphorus, aluminum and arsenic are typically found in silicon sources used to make the silane or halogenated silane. Trace amounts of heavy metals can be picked up from the processing equipment. Carbon may be introduced from gasketing materials, lubricants, inerting gases and the like. Of course, with halogenated silanes, the halogen is an impurity that may be present in the polycrystalline silicon.
With some impurities, the conversion to unicrystalline silicon can effect some purification. For instance, the phosphorus content can be reduced by the conversion to unicrystalline silicon and further reduced by drawing the crystal. The distribution coefficient (segregation coefficient) of impurities between the melt and crystal also plays a role. Usually, the incorporation of the impurity into the crystal structure will be at a lesser content than the concentration of the impurity in the melt. However, as the crystal is drawn, the impurity becomes more concentrated in the melt, leading to more impurity being incorporated into the later formed portions of the crystal. Neither of these techniques are fully satisfactory to alleviate loss of resistivity and lifetimes due to impurities, and can result in shortcomings such as inducing contamination through multiple drawings of the unicrystalline silicon or loss of yield from the polycrystalline silicon. Furthermore, for impurities such as boron which has a distribution coefficient of about 0.8, these techniques are not viable for removing the contaminants.
Accordingly, highly pure polycrystalline silicon is a desired feedstock to make unicrystalline silicon with high resistivities and long lifetimes. Typical commercial polycrystalline silicon of float zone grade (i.e., higher purity than the grades used for the Czochralski technique) ranges in boron content from about 20 to 30 parts per trillion (atom basis) (ppta) and phosphorus, 30 to 100 ppta, after float zoning to produce unicrystalline silicon. Despite efforts being applied by various researchers in the field, no commercial polycrystalline silicon has been available which will provide a unicrystalline silicon made by the float zone technique having less than 20 ppta boron and 20 ppta phosphorus with high resistivities, e.g., greater than 10,000 ohm cm, and long lifetimes, e.g., in excess of about 10,000 microseconds (ASTM F-28-75, reapproved 1981). Not only will the presence of impurities adversely affect the resistivity and lifetime of the unicrystalline silicon, but also defects in the crystal will reduce resistivity and lifetime. While the defects can be reduced by multiple drawings of the unicrystalline silicon, e.g., repeated float zoning operations, a trade-off exists due to the contamination of the unicrystalline silicon inherent in the float zoning process.
Heretofore, workers in the field have proposed numerous processes for preparing polycrystalline silicon. The Siemens process, which appears to be the most widely practiced commercial process, involves the decomposition of trichlorosilane at about 1000.degree. C. The silicon is deposited on heated rods, usually electrified to maintain the temperature. The deposition of silicon from SiX.sub.4 or SiHX.sub.3 (X is Cl, Br, I) is disclosed in U.S. Pat. Nos. 3,012,862 and 4,054,024. Silicon tetrachloride (decomposition temperature of about 1200.degree. C.) and silicon tetraiodide (decomposition temperature of about 900.degree. C.) have also been proposed. Several manufacturers decompose silane (temperatures of 400.degree. to 900.degree. C.) to form polycrystalline silicon. The decomposition can be on a heated rod or in a fluid bed. Another proposal has been the decomposition of tribromosilane at 600.degree. to 800.degree. C. to form polycrystalline silicon. Dichlorosilane can also be decomposed to silicon.
Some attention has been paid to the nature of the polycrystalline silicon product. U.S. Pat. No. 4,255,463 is directed to a method for the decomposition of trichlorosilane (or other silicon halogen compound) under conditions that provide a fine-crystalline surface. The patentees state at column 1, lines 38 to 44:
"It has been found that during the production of polycrystalline silicon rods, a coarse crystalline growth occurs, at times, leading to the occurrence of considerable crystal lattice faults during subsequent production of monocrystalline rods from these polyrods in crucible free floating zone melting process." PA1 "With the method according to the invention, it has been possible to reduce the size of the crystallites to about 1/3 as compared to the hereinaforementioned conventional method."
The patent, however, provides no details of the physical properties of the polycrystalline silicon other than at column 4, lines 3 to 6:
Commercially available polycrystalline silicon made by the decomposition of trichlorosilane typically has large grain sizes readily observable by optical microscopy, e.g., 10,000 to 250,000 angstroms. Earlier, Dyer, in U.S. Pat. No. 3,540,871 proposed a halosilane decomposition process in which the temperature in the decomposition reactor is controlled to affect the substrate upon which the silicon is being grown. This process is said to promote defects, e.g., twins, and promote polycrystalline growth. Commercially-available polycrystalline silicon made by the decomposition of silane generally has crystallite sizes ranging from about 100 to 500 angstroms.